<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <div id="main">
        <h3>h3</h3>
        <span id="span">span1</span>
    </div>
</body>
</html>
<script>
    var span=document.getElementById("span");
    
    function myPre(ele){
        var pre=ele.previousSibling;
        while(pre&&pre.nodeType!==1){
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        return pre;
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    var res=myPre(span);
</script>